1. Field of the Invention
The invention relates to metal film deposition. More particularly, the invention relates to anodes used in metal film deposition.
2. Description of the Prior Art
Electro-chemical plating (ECP), previously limited in integrated circuit design to the fabrication of lines on circuit boards, is now used to fill features in substrates such as vias and contacts. ECP, in general, can be utilized with a variety of processes. One process including ECP comprises depositing a barrier diffusion layer over the feature surfaces of the wafer by a process such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). A conductive metal seed layer is then deposited over the barrier diffusion layer by a process such as PVD or CVD. A conductive metal film (e.g. copper) is then deposited on the seed layer by ECP to fill the structure/feature. Finally, the deposited metal film is planarized by a process such as chemical mechanical polishing (CMP), to define a conductive interconnect feature.
Deposition of a metal film on a seed layer during electroplating is accomplished by electric voltage biasing the seed layer on a substrate relative to an anode. During ECP processing, both the seed layer and the anode are contained in electrolyte solution in an electrolyte cell. The seed layer is electrically biased to attract metal ions within the electrolyte solution to be deposited on the seed layer.
Those anode surfaces that are exposed to electrolyte solution typically chemically react with the electrolyte solution, and eventually degrade. Particulate matter from degraded anodes may also be dispersed into the electrolyte solution. If the particulate matter contacts the seed layer, the seed layer can become physically distorted, chemically) altered, and generally contain irregularities. Such irregularities can effect the electrical characteristics (such as electric current density) of the seed layer.
In ECP systems an electric field is established between an anode and a seed layer on a substrate during the metal film deposition. To enhance the metal film deposition on the seed layer, an electrolyte solution fluid flow is established, for example, from below the anode, through or around the anode, towards the substrate. The chemical reaction between the anode and the electrolyte solution is enhanced by the addition of electricity applied to the anode. This enhanced chemical reaction supplies metal ions into the electrolyte solution from the metal forming the anode. The combination of the electric field established from the anode to the seed layer on the substrate in combination with the fluid flow in the electrolyte cell, acts to transport metal ions from the anode toward the seed layer on the substrate. Generation of metal ions by the anode into the electrolyte solution, in addition to the formation of such particulate byproducts as anode sludge, acts to degrade the surface of the anode. After a certain period, this degradation of the metal surface of the metal of the anode produces an uneven upper anode surface.
During the ECP process, it is desired to maintain the anode surface facing the substrate in a planar configuration, with the face of the plane oriented substantially parallel to the seed layer. Any anode surface deviation from being planar results in a variation of spacing between different points on the upper surface of the anode from the nearest locations on the seed layer. The electric resistance of any point on the anode to the nearest seed layer point via the electrolyte solution varies as a function of distance through the electrolyte solution. A shorter distance between any particular location on the upper surface of the anode to its nearest location on the seed layer through the electrolyte Solution typically results in a decreased electrical resistance between the anode surface location to the seed layer surface because electrolyte solution resistance varying as a function of Ohms law.
Since establishing, a substantially uniform electric current density across the surface of the seed layer on the substrate enhances the uniformity of metal film deposition across the seed layer, a variation of the resistances between the various anode surface locations and the seed layer results in a variation of the electric current density across the seed layer on the substrate. Such variations in the electric current density across the seed layer results in variations in metal film deposition across the surface of the substrate. In the metal film deposition process, it is desired to maintain the metal film deposition across the surface of the seed layer as uniform as possible to provide uniform electrical characteristics of the metal film following the metal film deposition. It thus becomes difficult to maintain seed layer plating on the substrate when the upper surface of the anode assumes an irregular, non planar, configuration as a result of the chemical reaction between the upper surface of the anode and the electrolyte solution as often occurs with extended anode use.
Therefore, it is desired to provide a system by which those portions of the anode that face a substrate, immersed in the electrolyte solution, are maintained substantially planar even following extended use in a configuration to enhance the resultant uniformity of the electric current density across the seed layer. Such electric current density enhancement also enhances uniformity of the metal film deposition rate across the seed layer.
This invention generally relates to an anode configured to be used within a metal film plating apparatus. The anode has a substantially planar electric field generating portion and an electrolyte solution chemical reaction portion. The planar electric field generating portion is coated with an inert material that is impervious to the electrolyte solution. In one embodiment, the anode is formed as a perforated anode. In one aspect, the planar electric field generating portion is formed contiguous with the electrolyte solution chemical reaction portion. In another aspects, the planar electric field generating portion is formed as a distinct member from the electrolyte solution chemical reaction portion.